I just sent my ATV3 back to Apple and they are sending out another unit. I had 5.1 on it and hotline had me revert back to 5.02 through iTunes. In my case, the WiFi worked but when Cat5 cable was plugged in, I wasn't getting any connection. Although the hotline thought the cause was the new version, I think the onboard network was dead.

After reverting, date and time screen came up and remote was no longer functioning. At least they were aware of the issue and replaced it quite fast.